1. Field of the Invention
The present invention relates generally to the art of batteries and more particularly to sealed, high-power lead-acid batteries of a modified bipolar nature. Still more specifically, in the most preferred embodiment, the invention relates to a battery which includes a expanded metal grid inserted through slots in a non-conductive film and folded therearound prior to pasting to form individual quasi bipolar battery elements.
2. Description of the Prior Art
Bipolar batteries have been known for many years and typically include an electrode pair constructed such that positive and negative active materials are disposed on opposite sides of an electronically conductive plate. Batteries prepared from bipolar electrodes typically comprise a series arrangement of the electrodes, with separate elements for the terminal electrodes, which are typically monopolar in nature.
Several advantages of bipolar electrodes are noted when they are compared to monopolar batteries, the latter being typified by automotive batteries. In monopolar systems, electrodes of like polarity are usually connected by means of a lead lug, with monopolar cells connected in series using intercell welds. In an ideal bipolar configuration, the cell-to-cell discharge path is comparatively shorter and dispersed over a larger cross-sectional area, thus reducing the ohmic resistance and improving power capabilities. Moreover, operating voltages of bipolar batteries should be considerably higher due to the lower resistance, providing the capability of producing high voltage batteries in relatively small, light-weight packages.
Lead-acid batteries are attractive candidates for bipolar construction because of the high power capabilities known chemistry, excellent thermal characteristics, safe operation and widespread use. The key component, however, in the development of lead-acid bipolar batteries has been the bipolar substrate construction. That element must be non-porous and must be able to withstand the corrosive environment within the lead-acid battery enclosure.
Numerous prior bipolar systems have involved the use of a variety of substrate materials, including conductive oxides dispersed in resin binders, high surface area conductive carbons dispersed in binders, and combinations of the foregoing. The problems of constructing suitable substrates using particles dispersed in a binder include properly wetting the conductive material, achieving proper sheet formation and sealing of this sheet in a battery environment. Fillers must be suitably conductive, must exhibit no porosity when dissolved in a carrier, must have low solubility, and must be stable at the lead and lead oxide electrode potentials which exist on the opposites sides of the substrate. The filler materials also cannot act as catalysts for oxygen or hydrogen evolution in these sealed battery environments.
Lead substrates have been used in bipolar designs, but are not particularly useful because of the weight of the substrate when compared to filled resin substrates. Further discussion of efforts to develop bipolar substrates can be found in U.S. Pat. No. 5,017,446, issued May 21, 1991 to Reichman et. al for "Electrodes Containing Conductive Metal Oxides", and U.S. Pat. No. 4,173,362, issued Dec. 22, 1992 by Tekkanat et. al and entitled "Composite Substrate for Bipolar Electrodes."
Several batteries which are quasi-bipolar in nature are described in earlier patents. In Oakley, U.S. Pat. No. 3,723,181, issued Mar. 27, 1973 for "Duplex Electrode Construction Using Continuous Electrically Non-Conductive Carrier Strip", the electrode is constructed on an electrically non-conductive carrier strip provided with holes and a conductive material is provided on both sides of the strip and is electrically connected through the holes. Positive and negative electrode deposits are made over the conductive material on both sides of the carrier strip.
In Rippel, et al., U.S. Pat. No. 4,353,969, issued Oct. 12, 1982 for "Quasi-Bipolar Battery Construction and Method of Fabricating", lightweight batteries include continuous strips of thermoplastic material and a plurality of electrically isolated lead strip arrays about which the strips are folded to provide pleated bipolar plates. The strips are sealed along their edges for receiving separator plates and so that electrolyte may be maintained within the cells. The active material for the battery is carried in scrim fabric and the entire assembly is held under compression by exterior resilient means.
A "Secondary Battery" is described in McCullough, et al., U.S. Pat. No. 4,830,938, issued May 16, 1989, and includes a series of cells defined by electrodes having a body length sufficient to be inserted through an insulating wall into adjacent cells, the electrode extending from the bottom of one cell, through the insulator at a location near the top of the electrode and into the next cell. The electrodes are electrically conductive carbonaceous material having defined properties which act as energy storing material.
In U.S. Pat. No. 4,048,397, issued Sep. 13, 1977, Rothbauer describes a "Method and Apparatus for Interconnecting Stacked Electrodes of Batteries." In this device, the batteries are interconnected by means of a pair of conductive bands insulated from one another and bent into a zig-zag formation. Each band overlays an electrode of one polarity, thus performing the function of a separator as well as a terminal conductor. In the disclosed embodiment, the conductive strips are made from a material such as stainless steel.
Another battery system which includes folds in certain of the components is described in U.S. Pat. No. 4,761,352, issued Aug. 2, 1988 to Bakos, et al. for "Accordion Folded Electrode Assembly." In this device, the anode, cathode and separator are laminated together in an accordion fold to form electrochemical cells. The folding is carried out to eliminate manufacturing steps involving the insertion of plates into pleats of a zig-zag anode and the need to make series electrical connections between individual cathode plates.
All of the foregoing systems suffer one or more sealing, fabrication or performance drawbacks. Furthermore, those systems which involve the folding of electro-active material over an insulating separator may suffer corrosion problems and they involve lengthy current collection flow paths and increased ohmic resistance. A need exists in the art for a high-power battery having low internal resistance, large active areas, low volume and weight, long life cycles and reduced failure mechanisms. The need is compounded by the failure of the art to successfully overcome existing and well documented problems with bipolar systems, including primarily the development of suitable substrates.